Flowmeters and gas flow regulation

Valves
The needle valve is the most common means of regulating gas flow rate. As the valve is opened, the orifice around the needle becomes larger and flow increases. The valve cartridge itself is usually removable so it can be replaced if it is damaged.

The valve must not be over-tightened--this will drill out the orifice and cause it to become incompetent. Some valves, such as are found on most medical anesthetic machines, incorporate a stop to prevent the valve being over-tightened. The valve control knob is usually color-coded. In addition, oxygen flowmeter knobs frequently have fluted edges to distinguish them from those of other gases.

Flowmeters
Flowmeters on modern anesthetic machines consist of a tapered glass tube containing a bobbin or ball which floats on the stream of moving gas.

As the gas flow rate increases, the float is carried further up the tube, so indicating the flow rate.

• Flowmeters are specifically constructed for each gas, since the flow rate depends on both the viscosity and density of the gas.
• Only the correct tube and bobbin or ball can be used to repair broken flowmeters.
• Since the bobbin floats in the gas stream, flowmeters will only function correctly if the tube is vertical.
• Flowmeters will not function correctly if the tube is cracked.

Bobbins and balls
Bobbin flowmeters are more accurate, but more expensive, and should be read from the top of the bobbin (the shape of the bobbin is designed to optimize gas flow, and has nothing to do with being a 'pointer'). Ball float types should be read from the middle of the ball.

Bobbin flowmeter, reading 2 l/min	Ball-float flowmeter, reading 2 l/min

Inaccuracy in flowmeters
May be due to:

• The tube not being vertical.
• Back-pressure from, for example, a ventilator.
• Static electricity causing the float to stick to the tube.
• Dirt causing the float to stick to the tube.

Sequence of flowmeters
If several flowmeters are mounted together and a leak develops in one flow tube, a higher proportion of the gas coming from the upstream flowmeter will tend to be lost through the leak. If the oxygen flowmeter is upstream, this will tend to deliver a hypoxic misture to the patient.

Oxygen flowmeters should, therefore, always be mounted downstream of any other flowmeters, although there is no international agreement on this position.

Nitrous oxide/oxygen proportioning flowmeters

Owing to the potential hazards of delivering hypoxic gas mixtures to patients, a number of manufacturers developed nitrous oxide/oxygen proportioning flowmeters. In these, the separate oxygen and nitrous oxide control knobs are replaced with a single flow control and a proportioning valve that allows adjustment of the output oxygen concentration from 30% to 100%.
     Other machines have retained the separate oxygen and nitrous oxide flow control knobs, but these are linked mechanically or pneumatically so that it is impossible to deliver less than a specified oxygen concentration (typically 25% to 30% oxygen).

Other flowmeters

Turret-type flowmeters, in which the gas leaves the flowmeter from the bottom of the unit, are usually used to deliver oxygen in intensive-care situations, but may be found on some anesthesia machines (e.g. older Dräger models).
Acrylic-bodied flowmeters are fitted on some less expensive machines. These have a tendency to crack unless they are used with care.
The Heidbrink flowmeter is found on some older Ohio anesthesia machines, as well as Pitman-Moore 960 and 970 machines. The gas flow is read from the top of the vertical bar.
Bourdon flowmeters are found on flow-limited regulators (for delivery of oxygen directly to patients) but are not usually used on anesthetic machines. They consist of an adjustable pressure supply, a pressure gauge (calibrated in units of flow) and a special orifice. As the pressure is increased, flow through the orifice increases, which is reflected by the gauge.     One problem with this type of flowmeter is that if the orifice becomes partially blocked, the flowmeter will over-read (since, for any given pressure, the flow will be less because of the increased resistance of the orifice), and vice versa.

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